1. Technical Field
The present invention relates to a sheet transporting device, which includes a transportation roller that transports a sheet to a transportation path by rotating in a forward direction; a discharge roller that discharges the sheet in the transportation path by rotating in a forward direction and that feeds the sheet to the transportation roller via a reversing path by rotating in a reverse direction; and a motor that is a source of driving force for the transportation roller and the discharge roller.
2. Related Art
Image reading apparatuses having an automatic document feeder, which is an example of a sheet transporting device, are known, such as copiers, facsimiles, and scanners. A known automatic document feeder that is provided in such image reading apparatuses transports a document sheet to a transportation path in which an image reading unit is provided. Thereafter, when the document sheet comes to the position where the document sheet can be discharged, the document sheet is reversely fed through a reversing path by using a switching-back structure. Thus, the document sheet is turned upside down and again transported to the position where the image reading is performed. For example, see JP-A-2001-354328. By providing such an automatic document feeder in a copier, a facsimile, a scanner, or the like, both sides of the document sheet can be automatically read. Moreover, recording apparatuses such as printers capable of performing automatic duplex printing are examples of known electronic apparatuses provided with a sheet transporting device using a similar switching-back structure.
In the above-described automatic document feeder, when the document sheet is transported in the direction of transportation, transportation rollers (for example, a pair of registration rollers in JP-A-2001-354328) and discharge rollers (for example, a pair of discharge rollers in JP-A-2001-354328) are rotated in forward directions.
When the document sheet is switched back, the discharge rollers are rotated in a reverse direction. The document sheet that is reversely fed to the reversing path has been turned upside down, and is nipped between the transportation rollers for a second time. From this time, at least until the document sheet passes the discharge rollers, it is necessary either to continue the rotation of the discharge rollers in the reverse direction or to release the driven roller of the discharge rollers by using a release mechanism that is actuated by a solenoid or the like, while keeping the rotation of the transportation rollers in the forward direction.
Furthermore, in order to remove skew of the document sheet by using the transportation rollers, it is necessary to temporarily stop the rotation of the transportation rollers only, temporarily reverse the direction of rotation of the transportation rollers, or the like. By such an operation, the attitude of the document sheet relative to the direction of transportation is corrected and the skew of the document sheet relative to the direction of transportation is removed. In image reading apparatuses having an automatic document feeder, such a skew-removing operation is necessary in order to achieve high accuracy of reading, because there may be a relatively large skew when the back side of the document sheet is read after the document sheet is reversed.
For the reasons described above, a known automatic document feeder has a plurality of motors as the source of driving force, and as a result, the cost thereof increases.